In recent years, there has been an increased interest in the use of indium phosphide (InP) for the manufacture of semiconductor devices. Because of the chemical and physical properties which InP possesses, the use of InP in semiconductor devices expands the range of devices which can be fabricated beyond those which were limited to more commonly used materials such as gallium arsenide (GaAs) or silicon. In particular with respect to microwave devices, InP devices have higher speed, higher power, higher microwave power conversion efficiency, and lower noise than GaAs devices. In addition, InP devices permit working with larger geometries for a given frequency than GaAs devices, thus avoiding the increased cost and difficulty associated with the high resolution lithographic processes required for the fabrication of devices with submicrometer dimensions. The InP used in semiconductor devices is usually in the form of either an epitaxial layer deposited on a substrate or a substrate wafer formed from bulk InP which is subsequently sliced.
In order to grow epitaxial layers of InP, a vapor phase epitaxial process has been used, in which an epitaxial layer is grown on the surface of a substrate from reactants which are in the vapor phase, as described, for example, by R. O. Fairman, M. Omori, and F. B. Fank in the publication entitled "Recent progress in the control of high-purity VPE InP by the PCl.sub.3 /In/H.sub.2 technique," in the Institute of Physics Conference Serial Number 336, 1977, page 45. However, a vapor phase epitaxial process has the inherent disadvantage that dopants, such as zinc and other p-type dopants, having a high vapor pressure are not suitable for vapor phase doping since they are difficult to control. Furthermore, materials grown by vapor phase epitaxy have been found to have a relatively high concentration of defects.
As an improvement upon vapor phase epitaxial processes, epitaxial layers of compounds of Group III and Group V elements have been grown from the liquid phase to provide layers with high reproducibility and low carrier concentrations. It has also been shown that, for III-V compounds other than InP, a liquid phase epitaxial (LPE) process can use dopants with high and low vapor pressure which are not suitable for vapor phase epitaxial processes. One liquid phase epitaxial process which is directed to the growth of GaAs layers is disclosed in U.S. Pat. No. 3,994,755, assigned to the present assignee, wherein a selected saturated solution is controllably cooled below its saturation temperature at a predetermined rate to epitaxially deposit a thin GaAs layer of a GaAs substrate. While this type of process has been generally satisfactory, in some cases the problem of thermal shock at the substrate-solution interface has been encountered when the substrate is dipped directly into the molten material, and this, in turn, produces nonuniform nucleation and crystal growth in the epitaxial growth process.
In order to overcome the above-mentioned problem of thermal shock, a horizontal slide bar system, or limited melt system, has been used and is disclosed, for example, by L. R. Dawson in an article entitled "Near Equilibrium LPE Growth of GaAs-Ga.sub.1-x Al.sub.x As Double Heterostructures," in the Journal of Crystal Growth, Vol. 27, (1974), pp. 86-96. However, in such a horizontal slide bar system, relatively large amounts of impurities are frequently introduced into the melt as a result of the horizontal orientation and the overall geometry of the system. Thus, the purity of the crystals grown in a horizontal slide bar system is difficult to optimize. In addition, crystals grown in a horizontal slide bar system may be nonuniform in composition, since it is difficult to establish solution homogeneity without stirring, which the horizontal slide bar system cannot provide.
In order to overcome these and other disadvantages of the horizontal slide bar system and to eliminate the previously discussed thermal shock problem of the vertical liquid phase epitaxy processes, a new method was developed and disclosed in U.S. Pat. No. 4,026,735, assigned to the present assignee, wherein a GaAs substrate is placed in a nonreactive container, the container is immersed in a solution and the substrate is retained in the container and shielded from the solution until thermal equilibrium is established between the solution and the container. Then, the solution is cooled to slightly below its saturation equilibrium temperature, the substrate is exposed to the solution, and epitaxial growth on the substrate occurs.
One problem associated with all of the above-mentioned prior liquid phase epitaxial deposition processes is that silicon from the quartz reaction vessel and associated tubing in the deposition apparatus becomes dissolved in the selected saturated solution and is subsequently incorporated in the epitaxially grown layer. In addition, silicon impurities incorporated in a grown crystal may have their origin in the starting materials used to prepare the growth solution. Since silicon is an n-type dopant in InP, the incorporation of silicon impurities in InP produces a high n-type impurity level that increases the conductivity of the InP. Since high purity, high resistivity InP is required for use in certain types of semiconductor devices, the need for minimizing undesired n-type impurities that would increase conductivity is manifest. The impurity level of silicon in InP is particularly high since the segregation coefficient for silicon in InP growth from solution is &gt;10 for temperatures of approximately 700.degree. C. (The segregation coefficient is defined as the concentration of the impurity in the host solid divided by the concentration of the impurity in the host solution.) The effect of silicon impurities in a grown crystal of a given material depends on the segregation coefficient for silicon in that material. The higher the segregation coefficient, the more noticeable the effect of silicon impurities in a grown crystal of that material.
One mechanism by which the silicon becomes dissolved in the saturated epitaxial growth solution is suggested in the equations shown below. Equation (1) shows the reaction of the hydrogen ambient gas with SiO.sub.2 solid(s) from the quartz or other silicon-containing reaction vessel or crystal growth apparatus tubing to form SiO vapor (v) and water. The SiO vapor can be carried to the surface of the crystal growth solution, where the SiO reacts further, as shown in Equation (2), with hydrogen to form silicon, which dissolves in the saturated solution, forming Si(d). The concentration of water formed in Equations (1) and (2) depends on the temperature at which the reaction occurs and the rate of the reaction at that temperature. EQU SiO.sub.2 (s)+H.sub.2 (v).revreaction.SiO(v)+H.sub.2 O(v) (1) EQU SiO(v)+H.sub.2 (v) .revreaction.Si(d)+H.sub.2 O(v) (2)
In one attempt to overcome this problem of a silicon background donor in epitaxial InP grown by a chloride vapor transport method, N. C. Hales and J. R. Knight, in the publication entitled "The Electrical Properties of Vapour Epitaxial Indium Phosphide Grown in the Presence of Oxygen," in the Journal of Crystal Growth, Vol. 46, 1979, pages 582-584, have reported that the addition of small quantities of oxygen can suppress the residual donor impurity (i.e., silicon).
A similar approach to reducing the silicon background donor in InP grown by liquid phase epitaxy was reported by S. H. Groves and M. C. Plonko in the publication entitled "LPE growth of nominally undoped InP and In.sub.0.8 Ga.sub.0.2 As.sub.0.5 P.sub.0.5 alloys," Institute of Physics Conference Serial Number 45, pages 71-77. By the process of Groves and Plonko, crystals of InP are grown in a horizontal, fused-silica growth tube with a graphite slider. Oxygen was added to the flow of hydrogen to form water, which is believed to convert silicon impurities in the growth solution to SiO.sub.2. A baking period of 17 or more hours was required for this process to be effected.
One specific area in which the growth of epitaxial layers with defined impurity levels is necessary is in the fabrication of Gunn diode or oscillator devices which consist of a series of two or more uniformly doped layers with differing impurity concentrations. (The manner in which a Gunn diode device funtions is described by P. J. Bulman, G. S. Hobson and B. C. Taylor in the book entitled "Transferred Electron Devices," Academic Press, New York, 1972, pages 1-10.) The vapor phase epitaxial process discussed above has been widely used to form Gunn devices, as discussed, for example, by R. J. Hamilton, Jr., R. D. Fairman, S. I. Long, N. Omori, and F. B. Fank, in the publication entitled "InP Gunn-Effect Devices for Millimeter-Wave Amplifiers and Oscillators," in the IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-24, No. 11, November 1976, pp. 775-780. Vapor phase epitaxy is especially suited to the fabrication of Gunn devices, since such a process is readily adaptable to rapid changes in n-type dopant profiles, as is required to form multiple epitaxial layers with differing dopant concentrations. However, such a process produces devices with relatively high defect densities, which degrade the performance of the device.
It is the alleviation of these prior art problems associated with producing an epitaxial layer of a III-V material having a controlled level of impurities therein to which the present invention is, in part, directed.
The problem of silicon contamination of InP applies to InP grown in bulk as well as to epitaxially grown InP. Indium phosphide has been grown in bulk for subsequent slicing into substrate wafers. One example of such a bulk growth process is the Czochralski method, or vertical pull method, as described, for example, by Y. Seki, H. Watanobe, and J. Matsui, in the publication entitled "Impurity effect on grown-in dislocation density of InP and GaAs crystals," Journal of Applied Physics, Vol. 49, 1978, page 822, in which a seed crystal is immersed in a melt and is then rotated while being slowly withdrawn from the melt. The heating of the melt and the rate of pulling the crystal from the melt are controlled to provide a crystal of the desired size and shape. However, if either the reaction vessel which contains the growth solution of melt or the associated parts of the crystal growth apparatus are made of quartz or of a material which contains silicon, the reactions of Equations (1) and (2) discussed above will occur. Silicon will become dissolved in the growth solution or melt and will be incorporated as an undesired impurity in the InP crystal grown. In addition, silicon impurities from the starting materials may be present. The present invention also seeks to overcome this problem of silicon impurities in bulk-grown InP, as well as in the previously mentioned epitaxially grown InP.